The invention relates to dry-reed contacts using terminal elements having preferably a cylindrical metal core with a nickel and copper covering extending in an airtight manner from a metallic portion of the capsule over fusible beads of printed glass. A material is employed for the metal core which has an expansion coefficient matched to that of the printed glass used for the fusible beads of printed glass.
According to the technique heretofore employed for the manufacture of dry-reed contacts of the type referenced hereinabove, terminal elements are generally used having a metal core made from an iron/nickel alloy known under the trade name "Vacovit". The core is nickel-plated with a sheath thickness of about 5 micrometers and which is subsequently copper-plated with a sheath thickness of 2.5 micrometers. The expansion coefficient of the alloy for the metal core is matched to that of the printed glass used for the fusible beads of printed glass, so that no capillary cracks arise in the vitreous coating, which would endanger the gas density. These cracks usually result from unavoidable temperature variations in industrial applications of dry-reed contacts. Of the above mentioned coatings vapor-deposited or, preferably, electro-deposited on the metal core, the nickel sheath is applied to ensure an excellent vitreous coating, and the copper sheath facilitates the subsequent tin-plating of the portion of the terminal elements jutting out of the enclosing case.
The foregoing prior art technique has the drawback that the comparatively thin copper sheath diffuses mainly into the nickel coating during the sealing process when the terminal elements are placed in the capsule, so that at the end of the sealing process the surface of the terminal element has a very high nickel content. The presence of this high nickel content during the further manufacturing process, and particularly if the dry-reed contact has been stored a long time, leads to rust formation and makes it very difficult for the terminal elements to be tin-plated and for electrical lines to be soldered thereto later.
To overcome the foregoing problem it is obvious that one should attempt as a first step to reduce the diffusion time of the copper into the nickel coating. However, this remedy is inadequate, because the total time of the melting process cannot for obvious reasons (necessary preheating of the entire contact or at least of the melting zone, softening of the fusible bead or printed glass, etc.) be reduced arbitrarily.
Another remedy for overcoming the above shortcomings is to make the copper coating substantially thicker so as to obtain an adequate copper sheath on the surface of the terminal element, even if the melting process exceeds the minimum period. However, attempts in this direction with terminal elements covered by a comparatively thick copper coating (copper-clad wires customarily available in the trade) also yield negative results, because the above necessary reaction of the nickel coating could not take place so as to result in a gasproof glaze, and the surface remaining after the diffusion, which contained a large amount of copper, obviously did not yield the absolutely necessary gasproof glaze which can assuredly be obtained if the surface contains an adequate amount of nickel.
It is an object of this invention to solve the problems described hereinabove and to provide terminal elements for dry-reed contacts of the type mentioned hereinabove which ensure a hermetically sealed glaze and, insure the possibility of tin-plating with ease the terminal elements for subsequent soldering.